Dr. Horrible's Sing-Along Blog consists of three acts of approximately 14 minutes each. They were first released online in July 2008 as individual episodes, with two-day intervals between each release.

Dr. Horrible is filming an entry for his video blog, giving updates on his schemes and responding to various emails from his viewers. Asked about the "her" that he often mentions, he launches into a song about Penny, the girl he likes from the laundromat ("My Freeze Ray").

The song is cut short by his "evil moisture buddy" Moist, who brings up a letter from Bad Horse, the leader of the Evil League of Evil. The letter informs Dr. Horrible that his application for entry into the League will be evaluated, and that they will be watching for his next heinous crime ("Bad Horse Chorus").

The following day, Horrible prepares to steal a case of wonderflonium for his time-stopping Freeze Ray by commandeering the courier van using a remote control device. Penny happens to be on the same street ("Caring Hands"), and appears asking him to sign a petition to turn a condemned city building into a homeless shelter. However, the remote requires his attention, and he appears uninterested in her and her cause. As Penny leaves, Horrible is conflicted, but opts to steal the wonderflonium, telling himself that 'A man's gotta do what a man's gotta do' ("A Man's Gotta Do").

When Horrible remotely drives the van away, Captain Hammer appears and takes over Horrible's song, smashing the remote control receiver and inadvertently causing the van to veer towards Penny. Hammer pushes her out of the way (into a pile of garbage) just as Horrible regains control of the van and stops it, making it appear that Captain Hammer stopped the van with his bare hands. The two confront each other, with Hammer slamming Horrible's head on the van's hood, but Penny emerges to thank Hammer, making him forget about beating up Dr. Horrible. As Hammer and Penny serenade each other, Horrible glares at them while he makes off with the wonderflonium.

Dr. Horrible stalks Penny and Captain Hammer on their dates; Horrible sings of the misery of the human condition, and Penny sings of hope and the possibility of redemption ("My Eyes"). Penny and Horrible, known to her as Billy, begin to talk openly as friends.

On his blog, Horrible reveals that his Freeze Ray has been completed, and that he plans to use it the next day. The following post reveals that he has failed, as Hammer and the LAPD watch his blog, and they were ready for him. He then receives a phone call from Bad Horse and is reprimanded, saying that the only way to be inducted now is to commit an assassination ("Bad Horse Chorus (Reprise)"). Horrible is conflicted and can't decide on a victim, or even if he wants to commit a murder at all, even though the League will deny his application if he doesn't.

Billy chats with Penny over frozen yogurt, at the laundromat, about his problems ("Penny's Song"). As they grow closer, Penny mentions that Captain Hammer is planning to drop by. Billy panics and tries to leave, only to run into Hammer as he walks in. They feign ignorance on recognizing each other, but when Penny leaves them alone, Hammer reveals that he recognizes Billy as Dr. Horrible. Captain Hammer then taunts Dr. Horrible about his crush on Penny, happy to be taking the thing that Dr. Horrible wants most. It becomes obvious that Hammer doesn't really care about Penny but just wants to sleep with her to spite Horrible. Horrible decides to kill Hammer as his heinous crime for admission to Bad Horse's Evil League of Evil ("Brand New Day").

The city is abuzz with Captain Hammer's crusade to help the homeless and he is considered the city's new hero; Penny ponders her relationship with Captain Hammer, waiting at the laundromat to share frozen yogurt with an absent Billy; and Dr. Horrible goes into seclusion while obsessively constructing a Death Ray to kill Captain Hammer once and for all ("So They Say").

At the opening for the new homeless shelter, where a statue of Captain Hammer will be unveiled, Captain Hammer begins a speech of encouragement to the homeless, but it degenerates into selfish, condescending praise of his own excellence and relationship with Penny ("Everyone's a Hero"). Penny, embarrassed and disillusioned, quietly tries to leave as the crowd joins in singing Hammer's song, but they are interrupted by the appearance of Dr. Horrible, who uses the Freeze Ray on Captain Hammer, cutting his song short. Dr. Horrible taunts the shocked crowd and declares that they cannot recognize that Hammer's disguise is "slipping", and he reveals a second, more lethal laser gun: his completed Death Ray ("Slipping").

At last, Horrible aims the lethal weapon at the frozen form of Captain Hammer, but hesitates. At that moment the Freeze Ray unexpectedly fails, and a suddenly revived Hammer punches Horrible across the room. The Death Ray falls from his hands, damaging it. Hammer then picks up the Death Ray, turns it on Horrible, and triumphantly completes the final note of his prior song. However, ignoring Dr. Horrible's warnings, Hammer pulls the trigger and the damaged Death Ray misfires.[9] The weapon explodes in Hammer's hands, injuring him and causing him to feel pain, apparently for the first time in his life. He flees, a wailing wreck, asking for "someone maternal". Dr. Horrible realizes suddenly that he has succeeded in vanquishing his nemesis, but still having not committed the murder required by the League. Unfortunately, he discovers Penny slumped against a wall, impaled by shrapnel from the exploding weapon. Tragically, she dies in Horrible's arms, deliriously reassuring him that Captain Hammer will save them.

Dr. Horrible declares a Pyrrhic victory, with "the world [he] wanted, at [his] feet", seeing that her death is ironically the murder he required. In the aftermath, Horrible gains infamy and is free to commit additional crimes unfettered by Captain Hammer. Horrible becomes a member of the League, striding into a party in celebration of his induction, attended by Moist and the villains Pink Pummeller and Purple Pimp. Meanwhile, Captain Hammer is seen on a psychiatric couch sobbing to his therapist.

Dr. Horrible, donning a new outfit – red coat, black gloves and his goggles covering his eyes – takes his seat at the League, composed of fellow super-villains Tie-Die, Snake Bite, Professor Normal, Dead Bowie, Fake Thomas Jefferson, Fury Leika, and Bad Horse (who proves to be an actual horse). He addresses the camera, saying, "now the nightmare's real", and in working "to make the whole world kneel", that "[He] won't feel...". He completes the line "...a thing" in a final blog post as Billy, out of costume, looking numb and lost in the midst of his lab. ("Everything You Ever")

The musical contains 14 songs, including credits and reprises, but at the time of broadcast the song titles were not identified. The soundtrack was released through the iTunes Store on September 1, 2008[10] and was released on CD in the US on December 15, 2008.

Dr. Horrible's Sing-Along Blog Soundtrack made the top 40 Album list on release, despite being a digital exclusive only available on iTunes.[11]

Neil Patrick Harris as Billy/Dr. Horrible: An aspiring supervillain of the mad scientist variety with the catchphrase "I've got a Ph.D in Horribleness". He desires to become a member of Bad Horse's Evil League of Evil and use his inventions to take over the world and enact social change for the betterment of humanity. His socio-political beliefs include the paradoxical idea of autocraticanarchy: "The world is a mess, and I just... need to rule it". As Billy, he struggles to make a romantic connection with Penny.

Nathan Fillion as Captain Hammer: Dr. Horrible's archenemy. Hammer is self-centered and possesses super-human strength and near-invulnerability. He enjoys harassing Dr. Horrible even when the situation does not warrant it.

Joss Whedon funded the project himself (at just over $200,000[12]) and enjoyed the independence of acting as his own studio. "Freedom is glorious," he comments. "And the fact is, I've had very good relationships with studios, and I've worked with a lot of smart executives. But there is a difference when you can just go ahead and do something." As a web show, there were fewer constraints imposed on the project, and Whedon had the "freedom to just let the dictates of the story say how long it's gonna be. We didn't have to cram everything in—there is a lot in there—but we put in the amount of story that we wanted to and let the time work around that. We aimed for thirty minutes, we came out at forty two, and that's not a problem."[13] Some of the music was influenced by Stephen Sondheim.[14]

The production of the DVD included a contest, announced at Comic-Con, in which fans submitted a three-minute video explaining why they should be inducted into the Evil League of Evil. Ten winning submissions have been added to the DVD release.

The songs were recorded in a small studio set up in Joss Whedon's loft.[15]Dr. Horrible was shot in Los Angeles, with five locations on one soundstage, and one day of filming at the Universal Studios backlot street set, including the scenes of the van heist, and the giant Dr. Horrible crushing people beneath his feet.[16] Dr. Horrible's home, used for blogging scenes as well as the final party scene, is the house featured in the "Mad Scientist House" episode of Monster House.[17]

Whedon has said that the plan was to find a venue for the series that would enable it to earn its money back and pay the crew. This plan was to release the show onto the Internet, with an iTunes[18] release to follow. If the Internet and iTunes releases were successful enough, Whedon planned to greenlight an official DVD, which would include some "amazing extras".[19]

The musical's fansite launched in March 2008 (despite the official site containing nothing more than a poster at the time) and was the first place to publicly release the teaser trailer three months later on June 25, 2008.[20]

The episodes first aired at the Official Dr. Horrible website, hosted on Hulu, accessible internationally (unusual for the US-based service whose videos are typically not accessible to Internet users who reside outside the US) and free to watch (ad-supported). Act I premiered on Tuesday, July 15, 2008 – Act II followed two days later on July 17, and Act III surfaced on July 19. The episodes were taken offline on July 20 as planned, but became available again on July 28. The show was later (date unknown) restricted to the United States only.[21]

On October 10, 2009, all three acts were made available via iTunes for the UK and Australia. The film is also available from Amazon Video on Demand. On November 29, 2009 all three acts, both separate and together, were taken off Hulu. On February 22, 2010, the full feature became available to stream on Netflix, with all three episodes appearing sequentially. On March 20, 2014, it was removed from Netflix.[22]

On November 28, 2008, the Official Dr. Horrible website announced that pre-orders were being taken for the DVD. The following day Tubefilter reported that pre-orders of the Dr. Horrible DVD were "booming".[23]

The DVD was released exclusively at Amazon.com on December 19, 2008, in the United States[24] and on January 13, 2009, in Canada at Amazon.ca.[25]

The DVD is region free. During pre-ordering the Amazon page stated the discs would be manufactured on demand using recordable media. Although some customers report receiving DVD-R discs (identified by a purple data-side), most are receiving pressed discs.[24]

On June 2, 2009, a new release of Dr. Horrible's Sing-Along Blog was produced by New Video Group, which included the same materials as the Amazon DVD but was distributed through regular retail outlets.[26] A Blu-ray version was released on May 25, 2010, from New Video Group.[27]

Special features on the DVD and Blu-ray include Commentary! The Musical; commentary by the cast and creators; behind-the-scenes featurettes on the making of the movie and the music; the top 10 Evil League of Evil application videos from fans; and four easter eggs.[24]

The DVD and Blu-ray versions of Dr. Horrible include as an extra Commentary! The Musical, a commentary track comprising entirely new songs performed by the cast and crew, thus creating another whole musical on its own. The actors and writers sing various songs both as solos and with the entire company, playing versions of themselves. Commentary! is partly self-referential, and one of the co-creators, Zack Whedon, self-referentially comments that one song "wasn't even about the movie, it was about itself", which he claims is "like breaking the ninth wall".[28] As of January 8, 2010, Commentary! The Musical has been for sale on the iTunes Music Store.[29]

All proceeds from iTunes and DVD sales were dedicated to paying the cast and crew, who were not compensated at the time of production.[30] In November 2008, Joss Whedon blogged about Dr. Horrible's Sing-Along Blog and stated that due to its success they had been able to pay the crew and the bills.[31] Later, in 2012, Joss Whedon said during an interview with Forbes that "with the movie and the soundtrack and everything we’ve been able to do with it, we made over $3 million with it".[32]

"Dr. Horrible" was released as a special one-shot comic, detailing Dr. Horrible's origin story, with art by Joëlle Jones.[37]

All four stories were collected in Dr. Horrible, and Other Horrible Stories by Dark Horse Comics, in September 2010 (ISBN 978-1-59582-577-3). The collection also features an additional story about the Evil League of Evil.

On March 29, 2011, Dr. Horrible's Sing-Along Blog Book (ISBN 978-1-84856-862-4) was published by Titan Books. The book contains essays by Whedon, Fillion, Harris, Day, and Helberg; the complete shooting script; the script for Commentary: The Musical; and piano/vocal sheet music for Dr. Horrible's Sing-Along Blog.[38]

On August 29, 2008, the first authorized sing-along version of Dr. Horrible's Sing-Along Blog was hosted at Dragon*Con in Atlanta, Georgia, with showings reaching standing-room-only capacity. Felicia Day showed up to one of the showings.[39]

Stage productions of the show have become very popular at colleges and high schools.[40][41][42][43][44]

In October 2010, Ireland, an official production of the musical was performed in Trinity College Dublin by the college's drama society; The DU Players. The show took place from October 25–29, 2010.[45] In November 2010, Cult Classic Theatre in Glasgow, Scotland, performed the first official UK production of Dr. Horrible's Sing-Along Blog.[46]

In January 2011, Balagan Theatre performed Dr. Horrible's Sing-Along Blog at the ACT Theatre's Allen Theatre in Seattle. The production was reviewed by Broadway World.[47] A televised performance of a concert version of "Freeze Ray" can also be viewed.[48] In 2011, all rights to perform Dr. Horrible's Sing-Along Blog or Commentary! The Musical were no longer being granted by the creators.[49]

During the broadcast of the 2009 Primetime Emmy Awards ceremony, which was hosted by Harris, a speech by representatives of Ernst & Young was "interrupted" by a sketch featuring Harris as Dr. Horrible and Nathan Fillion as Captain Hammer, with cameos by Felicia Day and Simon Helberg.[52]

Fans of internet personality Doctor Steel noted similarities between Doctor Steel and Dr. Horrible, attracting the attention of national media. Dr. Horrible co-writer Maurissa Tancharoen responded, "All we have to say on the subject is we've never heard of Dr. Steel before... There's room for everyone in this party."[54]

Plans to make a sequel to Dr. Horrible's Sing-Along Blog are currently in development hell. In 2009, Joss Whedon announced that a sequel was in the works and that it might be presented as either another web series or as a feature film.[55] Nathan Fillion also said that he knew the title of the sequel, but was unwilling to reveal it at the time.[56]

In a New York Times interview in April 2011, Whedon said "We've got several songs near completion and we've got a very specific structure," and that the cast of Dr. Horrible have sung the songs at casual gatherings.[57]

In April 2015, Whedon stated in an update that "I was starting to feel like, I don't know if that’s [possible]. Everybody’s so busy, and we’ve all changed. I saw Neil do Hedwig [on Broadway], and went backstage, and the first thing he said was, 'When are we doing a sequel?!' I'm like, OK! Maybe that's still on! So it'll be interesting to see when we come up for air on it. Not sure. But god knows, we have the people. We have the technology. We can rebuild it."[61]